help with Vibration Please - Pontiac GTO Forum
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post #1 of 4 (permalink) Old 08-18-2017, 09:48 AM Thread Starter
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help with Vibration Please

1969 Pontiac tempest that PO installed a 1971 vintage 400 and THM350.

I get a vibration/shake at 45MPH which goes away above or below that number.

I currently have the driveshaft out and the car on jackstands. If I run the car in drive, I can feel the shake at 45 displayed on the speedometer. If I put it in Low ear, at about 25. If it is in park, I can feel it at a certain engine vibration. As I increase rpms it is smooth

I thought about flywheel, but what I read was that if it was flywheel, there would be vibrations at all speeds getting worse when rpms increase.

Could this be a transmission problem? any ideas what I should look for/check.

Also, I checked motor mounts and transmission mount and they area all good


Last edited by ajluzi1957; 08-18-2017 at 09:53 AM. Reason: update with additional information
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post #2 of 4 (permalink) Old 08-19-2017, 11:09 AM
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Ok, just to be clear --- you still have the vibration with the drive shaft removed? You also feel it in neutral/park?

If that's the case, then it *has* to be something from the converter/flex plate/transmission oil pump forwards - some part that still rotates even when the trans is in park. Best not run it much until you figure it out, as that's almost sure to hammer the main bearings out or worse (broken crank).

Suspicions: wrong flex plate i.e. the engine is internally balanced and still has the factory flex plate (or vice versa - engine is 'factory balanced' and someone installed a flex plate for an internally balanced engine).

Could also be anything between the trans oil pump and the harmonic damper.


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post #3 of 4 (permalink) Old 08-19-2017, 01:09 PM
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Something is amiss in the balance of the engine.

Check/inspect the fan/fan blades. Grab hold of it and wiggle it around and look for any looseness, bends, or cracks. Had a bent blade once that gave off an engine vibration - until it developed a crack and broke off at speed and really made the engine vibrate.

Check the vibration dampener at the front of the engine on the crank. These have an outer inertia ring and inner hub at the crank bonded together by rubber. The rubber deteriorates over time and the outer ring can slip or even fly off taking out some engine parts. IF this has slipped, it would create a vibration in the engine.

The next place to look will be the flexplate and torque converter-to-flexplate attachment bolts. Make sure the bolts have not loosened up on the torque converter and created some unwanted play.

It is possible the flexplate was somehow damaged during installation and is now causing the vibration. Short of pulling the trans, you could set-up a dial indicator and rotate the engine by hand a full revolution to check the flexplate for straightness. MAKE SURE THE BATTERY IS DISCONNECTED for safety's sake.
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post #4 of 4 (permalink) Old 08-19-2017, 02:53 PM Thread Starter
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Thank you both for the responses. Yes, I still get the vibration without the drive shaft. It occurs at about 45 in drive and 25 in low gear which makes me think it is ahead of the gearing like in the flex plate or torque converter that you both suggested.

I did change the harmonic balancer some time ago, so don't believe that is an issue.

I will check out the bolts first to make sure they are tight, and then if that doesn't work, I will change the flex plate since I think that can be done without removing the transmission.

I will let you know what happens.

Thanks again, Armand
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  Pontiac GTO Forum > The 1964-1974 Pontiac Tempest, Lemans & GTO > 1964-1974 Tempest, Lemans & GTO Undercarriage, Frame, Transmission and Differential Discussions.

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